Airplane landing gear are subjected to very high shock loads. To handle these loads it has become the accepted practice to use telescoping cylinders having both a pneumatic spring in the form of a gas to act in combination with and directly upon a hydraulic fluid having controlled flow as the cylinders interact. This serves to both absorb the loads and to dampen excessive rebounding which may bounce the airplane off the ground. A shock absorbing landing gear requires a stiff air spring with large hydraulic damping characteristics to handle the loads during touchdown, and it is desirable to have a soft air spring with small hydraulic damping characteristics after touchdown and during roll and/or while taxiing on the ground to give a more comfortable ride to passengers and or to improve the fatigue life of the airplane.
In U.S. Pat. No. 2,451,171 a second and collapsible air compartment is located in the hydraulic fluid. That secondary air is at higher pressure than the first or primary air and the second compartment collapses to act as a second pneumatic spring under heavy loads. Dampening of rebound is accomplished with flap valves. U.S. Pat. No. 3,888,436 uses high pressure secondary air to contain a piston, and under heavy loads the piston moves to provide a secondary pneumatic spring action. U.S. Pat. No. 2,735,674 uses high pressure secondary air to contain a piston which moves to provide a secondary pneumatic spring and in addition the piston contains a plunger to control openings and provide for fluid flow as the original fluid dampening flow is closed off under heavy loads.